F150 FAST IDLE

Hello,
My F150 started idling around 3000 rpm at start-up and stops. The local garage installed an air-idle control. This helped for awhile, then it started again, sporadically. Since they couldn't duplicate the problem at that time, they replaced the same control again for free. This solved the problem for several months. However, now at start-up it seems to be starting it again, although not as severe. It only runs at around 1500 rpm at start-up, then slowly idles back down to the typical 850. So far it hasn't over-idled at stops like it was before. Any suggestions other than replacing the air-idle control again? Thanks.

Too fast an idle speed. If an engine without computerized idle speed control is idling too fast and refuses to come down to a normal idle speed despite your best efforts to back off the carburetor idle speed screw or air bypass adjustment screw (fuel injection), air is getting past the throttle somewhere. Common leak paths include the carburetor and throttle body gaskets, carburetor insulator spacers, intake manifold gaskets, and of course, any of the engine's vacuum fittings, hoses and accessories. It is even possible that leaky O-rings around the fuel injectors are allowing air to leak past the seals. Another overlooked item can be a worn throttle shaft and a defective idle speed speed control motor/valve stuck in the extended (high idle speed) position/throttle position sensor. Also the throttle plate could be binding in its bore and kinked accelerator cable, coolant temperature sensor might not be operating properly misleading the computer that the engine is still cold and computer throwing fuel at it raising the idle speed.